CAREER: Engineering Ultra-Wide Bandgap III-Nitride Devices for Highly Efficient and Robust Electronics

The global demand for energy will likely significantly increase in the coming decades as more of the world gains access to electricity. This demand will need to be met without intensifying existing concerns over the environmental, societal, and economic impacts of climate change, which is influenced by the efficiency of power systems. Furthermore, since this efficiency is tied to losses associated with the semiconductor devices used in power electronics, the replacement of today's silicon-based components with a novel semiconductor technology must be investigated. Wide bandgap semiconductors are candidates, however ultra-wide bandgap (UWBG) semiconductors represent the next frontier. In particular, aluminum-rich aluminum gallium nitride (AlGaN) offers an exciting opportunity to explore a new generation of highly efficient and robust electronic devices. The robustness of these devices in the face of elevated temperatures or electromagnetic interference (EMI) also stands to improve system performance and reliability. Thus far, a combination of unresolved scientific questions and technological barriers have prevented AlGaN devices from reaching their full potential. The recent commercialization of aluminum nitride (AlN) wafers, however, permits the ideal properties of AlGaN films to be studied for the first time. Thus, the research objective of this CAREER project is to understand the operational physics of high voltage AlGaN devices and develop engineering solutions that will unlock their performance for power electronics where efficiency and robustness are key. The investigator is also committed to becoming a leading educator in the area of semiconductor devices via the use of team-based, laboratory-based methods and is passionate about ensuring equal access to STEM education/training. Three education/outreach tasks have been defined with the objectives of: (1) equipping undergraduates with marketable skills via hands-on research experiences that they can leverage for STEM careers, (2) engaging underrepresented minority groups to diversify the STEM workforce, and (3) increasing the public's scientific literacy surrounding semiconductors/electronics.

The objective of this CAREER project is to understand the performance limits of ultra-wide bandgap AlGaN-based power devices and establish solutions to deploy these devices in highly efficient and robust systems. The potential for Al-rich AlGaN to support large electric fields will be understood by experimentally extracting the impact ionization coefficients from avalanche photodiodes realized on native AlN substrates. The results will be used to minimize the drift region's contribution to the overall conduction loss. Since low resistance ohmic contacts to Al-rich AlGaN remain elusive, the surfaces and bulk of UWBG AlGaN will be engineered to obtain thermally stable contacts with reduced contact resistance. The surface barrier height will be reduced using both ex-situ and in-situ interlayers that compensate AlGaN's polarization charge. Tunneling contacts will also be sought by heavily doping the bulk AlGaN via Si implantation followed by effective activation annealing. The limits of high temperature operation will be explored. Next, p-type doping of AlGaN, in particular for buried and lateral regions, is a barrier towards advanced power devices. Thus, novel Crystal Heterogeneous Integration (CHI) will be investigated to combine p-type compound semiconductors with n-type AlGaN. The high voltage breakdown of heterogeneous PN junctions will be evaluated, as well as their response to light to interpret the resulting band structures and explore carrier transport across the junction. These research tasks will be leveraged to design, fabricate, and characterize high voltage (1-5 kV) AlGaN diodes, with a view to demonstrating the promise of AlGaN technology in future power systems. Their stability in the face of high temperature exposure will also be examined. To address the risk of EMI, the first phototransistor that heterogeneously integrates AlGaN with other compound semiconductors will be investigated for fast optical gating and high voltage blocking.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.